Transfer roller assembly

ABSTRACT

A transfer roller assembly having a transfer roller, is mounted adjacent a curved photoconductive surface of an electrostatographic machine so that the transfer roller is biased into contact with the surface with a minimal force sufficient only to maintain uniform contact therewith during machine operation; the roller is mounted so that it may move in a direction which is at least generally perpendicular to a tangent at the point of contact. The ratio of the radius of the photoconductive surface to that of the roller at the point of contact is not less than 3, and preferably in the range of 3 to 3.5.

United States Patent [191 Hoffman et al.

[ Aug. 26, 1975 TRANSFER ROLLER ASSEMBLY [73] Assignee: XeroxCorporation, Stamford,

Conn.

[22] Filed: July 2, 1973 [211 App]. No.1 376,086

3,751,156 8/1973 Szostak et a1 355/3 Primary Examiner-Mervin SteinAssistant ExaminerDouglas Salser 57 ABSTRACT A transfer roller assemblyhaving a transfer roller, is mounted adjacent a curved photoconductivesurface of an electrostatographic machine so that the transfer roller isbiased into contact with the surface with a minimal force sufficientonly to maintain uniform contact therewith during machine operation; theroller is mounted so that it may move in a direction which is at leastgenerally perpendicular to a tangent at the point of contact. The ratioof the radius of the photoconductive surface to that of the roller atthe point of contact is not less than 3, and preferably in the range of3 to 3.5.

2 'Claims, 4 Drawing Figures PATENTED AUG 2 6 I975 SIIZU 3 n5 3BACKGROUND OF THE INVENTION This invention relates generally toelectrostatography, but more particularly to a novel transfer rollerassembly for an electrostatographic machine.

In the practice of xerography as described in US. Pat. No. 2,297,691 toChester F. Carlson, a xerographic surface comprising a layer ofphotoconductive insulating material affixed to a conductive backing isused to support electrostatic images. In the usual method of carryingout the process, the xerographic plate is electrostatically chargeduniformly over its surface and then exposed to a light pattern of theimage being reproduced to thereby discharge the charge in the areaswhere light strikes the layer. The undischarged areas of the layer thusform an electrostatic charge pattern in conformity with theconfiguration of the original light pattern.

The latent electrostatic image may then be developed by contacting itwith a finely divided electrostatically attractable material, such as aresinous powder. The powder is held in the image areas by theelectrostatic fields on the layer. Where the field is greatest, thegreatest amount of material is deposited; and where the field is least,little or no material is deposited. Thus, a powder image is produced inconformity with the light image of the copy being reproduced. The powderis subsequently transferred to a sheet of paper or other transfersurface, and suitably affixed thereto to thereby form a permanent print.

The toner may be fixed to the paper by passing the latter between aheated roller and a second roller in pressure contact therewith, wherebythe toner becomes fused to the sheet of paper.

The image is generally transferred to the image receiving member (e.g.paper) by passing such member between the photoconductive surfaceincluding the developed image and a transfer roller in contacttherewith.

Roller electrode transfer systems employ DC electric fields to movecharged particles such as xerographic toner from first to secondsupporting surfaces, e.g., from a photoconductive surface to a sheet ofpaper. The purpose of this is to exert an electrical force on thecharged particles that moves them from the first to the second surface.Examples of bias roller transfer systems are described in US. Pat. Nos.2,807,233; 2,068,555; 3,043,684; 3,267,840; 3,598,580; 3,625,146;3,630,591; 3,691,993; 3,702,482; and 3,684,364, French Pat. No.2,065,390, German Application OLS 2,102,634 and British Pat. Nos.2,310,666 and 1,302,922.

Among the problems that arise in some bias transfer systems is one ofthe defects in copy quality, e.g., hollow characters which may resultfrom a sheet of paper being subjected to an excessive pressure as itpasses between the transfer roller and the photoconductive surfaceduring the transfer step or process.

Another copy quality problem which sometimes arises in bias transfersystems relates to undesired nonuniform densities in the transferredimage. This results from variations in pressure caused by excessiveshift of the transfer nip during the transfer step.

Dimensional variations in the machine and/or the paper affect thedynamic stability of the machine during operation, which in turn affectsthe operation of the transfer system, thus often resulting in one ormore of j the above problems.

Also, there is occasionally a problem with the paper wrapping itselfaround the transfer roller during the transfer step or operation.

BRIEF SUMMARY OF THE INVENTION It is also an object of the presentinvention to provide I a transfer roller assembly which minimizes oreliminates wrapping of a support member (e.g., a sheet of paper) aroundthe transfer roller during operation.

These and other objects are obtained by providing a transfer rollerassembly which is supported within an electrostatographic machineadjacent a photoconductive surface. The transfer roller assembly iscomprised of two rigid frame assemblies rotatably supporting springloaded pivoting arms in which the transfer roller is journalled forrotation; this structure provides the pressure contact between thetransfer roller and the photoconductive surface. The force biasing thetransfer roller toward the photoconductive surface is that minimal forcesufficient only to maintain uniform contact during operation of themachine. The transfer roller is mounted so that any shift of thetransfer roll during operation of the machine is in a direction which isat least generally perpendicular to a tangent at the point of contact.To eliminate or minimize the problem of paper wrapping itself around thetransfer roller, the ratio of the radius of the curved photoconductivesurface to the radius of the transfer roller at the point of contact isnot less than 3, and preferably in the range of 3 to 3.5. Electrically,for example, the transfer roller assembly of the present invention mayoperate as disclosed in US. patent application Ser. No. 309,562 filed onNov. 24, 1972, or in other suitable conventional manners.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the presentinvention as well as other objects and further features thereof willbecome apparent upon consideration of the following detailed disclosurethereof, especially when taken with the accompanying drawings whereinlike numerals designate like parts throughout.

FIG. 1 is a schematic sectional view of an electrostatic reproductionmachine embodying the principles of the present invention.

FIG. 2 is an elevational view of ,a transfer roller assembly embodyingthe principles of the present inventlon.

FIG. 3 is a sectional view of the transfer roller assembly of FIG. 2taken along the line 3-3 of FIG. 2.

FIG. 4 is a side view of the transfer roller assembly taken along theline 4-4 thereof of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION For a general understanding of theillustrated copier/reproduction machine in which the present inventionmay be incorporated, reference is had to FIG.

1 in which the various system componentsfor the machine areschematically illustrated.

A document D to be copied is placed upon a transparent support platen Pfixedly arranged in an illumination assembly, indicated generally by thereference numeral l and positioned at the left end of the machine. Lightrays from an illumination system are flashed upon the document toproduce image rays corresponding to the informational areas. The imagerays are projected by means of an optical system onto the photosensitivesurface of a xerographic plate in the form of a flexible photoconductivebelt 12 arranged on a belt assembly, generally indicated by thereference numeral 14.

The belt 12 comprises a photoconductive layer of selenium which is thelight receiving surface and imaging medium for the apparatus, on aconductive backing. The surface of the photoconductive belt is madephotosensitive by a previous step of uniformly charging the same bymeans of a corona generating device 13.

The belt isjournalled for continuous movement upon three rollers 20, 21,and 22 positioned with their axes in parallel relationship. Thephotoconductive belt assembly 14 is slidably mounted upon two supportshafts 23 and 24 secured to the frame of the apparatus with the roller22 rotatably supported on the shaft 23, and is rotatably driven by asuitable motor and drive assembly (not shown) in the direction of thearrow at a constant rate. During exposure of the belt 12, the portionexposed is that portion of the belt running between rollers and 21.During such movement of the belt 12, the reflected light images of suchoriginal document positioned on the platen is flashed on the surface ofthe belt to produce an electrostatic latent image thereon at exposurestation A.

As the belt surface continues its movement, the electrostatic imagepasses through a developing station B in which there is positioned adeveloper assembly, generally indicated by the reference numeral 15, andwhich provides development of the electrostatic image by means ofmultiple brushes 16 as the same moves through the development zone.

The developed electrostatic image is transported by the belt to atransfer station C wherein the transfer roller assembly 26 of thepresent invention is positioned and where a sheet of copy paper is movedbetween the nip formed by the assembly 26 and the belt 12. The paper ismoved at a speed in synchronism with the moving belt, and the transferof the developed image is accomplished solely by an electrical bias onthe transfer roller. There is also provided at station C a sheettransport mechanism generally indicated at 28 adapted to transportsheets of paper from a paper handling mechanism generally indicated bythe reference numeral 30 to the developed image on the belt at thestation C.

After the sheet is stripped from the belt 12, it is conveyed into afuser assembly, generally indicated by the reference numeral 31 whereinthe developed and transferred xerographic powder image on the sheetmaterial is permanently affixed thereto. After fusing, the finished copyis discharged from the apparatus at a suitable point for collectionexternally of the apparatus.

Further details regarding the structure of the belt assembly l4 and itsrelationship with the machine and support therefor may be found in thecopending application Ser. No. 102,312 assigned to the same assignee.

Referring now to FIGs. 2, 3, and 4, the transfer roller assembly 26 isillustrated, the assembly being comprised of a transfer roller 32,pivoting arms 34a and 34b, and rigid frame assemblies 36a and 36b. Thetransfer roller 32 is mounted for rotation about shafts 38 disposed inarms 34a and 3411, the latter being joumalled for rotation about shafts40 positioned within the assemblies 36a and 36b, respectively, such thatany perturbations of the transfer roller 32 against the photoconductivebelt 12 due to dimensional variations will not be deleterious to theimage transfer process. A support shaft 42 is mounted for rotation inthe assemblies 36a and 36b above and in parallel alignment with thetransfer roller 32, and a gear 44 is affixed to an end of the shaft 42for driving a cleaning brush member 48 through a suitable drive means incontact with the transfer roller 32. Bearings 50 are disposed on theshaft 42 outwardly of the assemblies 36a and 36b, and are positioned insupport assemblies of the machine indicated generally as 52 and 54.

Shafts 38 are journalled for rotation in the pivoting arms 34a and 34bas are the shafts 40, thereby permitting rotation of the transfer roller32 with respect to the arms 34a and 34b, and also permitting thetransfer roller 32, and assemblies 34a and 34b to rotate as a unit aboutthe shafts 40.

A shaft disposed above and in parallel relationship to the shafts 40, isjoumalled for rotation in the assemblies 36a and 36b and has theportions thereof extending beyond these assemblies provided with earns62 affixed thereto. A threaded rod member 64 is rotatably mounted by apin 66 to each of the assemblies 36a and 36b. The end of the threadedrod member 64 opposite the pin mount is provided with a centrally formedorifice (not shown) into which a rod member 'is positioned in slidingrelationship. A spring 68 is positioned about the rod member 65 betweeneach of the arms 34a and 34b and a nut 70 threaded on the rod 64. Alever 72 is connected to the shaft 60 to rotate the latter and is heldin operative relationship by a locking arm 74 extending from theassembly 36b.

When it is necessary to move the transfer roller 32 into anon-operational mode, i. e., where the roller is not in contact with thephotoconductive belt 12, the lever 72 is moved clockwise as viewed inFIG. 3 (counterclockwise as viewed in FIG. 4). This would be necessary,for example, when it is desired to change the photoconductive belt 12.When the lever is so moved, the cams 62 on each side of the assembly 26are moved against the arms 34a and 34b. For example, as can be seen inFIG. 4, cam 62 would rotate counterclockwise against the surface 74 ofarm 34b. A corresponding action would take place with respect to arm34a. Cams 62 would thus rotate arms 34a and 34b about the shafts 40. Itshould be noted at this point, that because of the location of shafts40, the transfer roller 32 moves in a direction which is at leastgenerally perpendicular to a tangent at the point of contactbetween thetransfer roller and the photoconductive belt 12. Thus, even during thetransfer process, any movement of the transfer roller 32 would be inthis direction. Heretofore, transfer rollers have been mounted so thatany movement of the roller during the transfer process was in adirection generally parallel to a tangent at the point of contact, whichresulted in poor copy quality as discussed above. It should also benoted at this point, that the springs 68 bias the transfer roller 32toward the photoconductive belt 12 with a minimal force which issufficient only to maintain uniform contact with the belt and the paperpassing between the belt and transfer roller during machine operation.

It will be understood that the transfer roller 32 will contact thephotoconductive surface in a manner to provide even contact therebetweensimilar to the shock absorbing action of the front end of a vehicle andthat the compressional forces of the springs 68 are adjustable byrotation of the respective nuts 78.

Another aspect of the present invention relates to the ratio of A, theradius of photoconductive belt 12 at the point of contact with thetransfer roller 32 to B, the radius of the transfer roller at thatpoint. This ratio should not be less than 3, and preferably is in therange of 3 to 3.5.

Other modifications of the present invention will occur to those skilledin the art upon a reading of the present disclosure which modificationsare intended to be included within the scope of this invention.

What is claimed is:

1. Apparatus comprising:

a. means defining a surface adapted to support a developed electrostaticimage, the surface being mounted for movement around a closed path;

b. a transfer roller assembly having a transfer roller in contact withthe surface, wherein the ratio of (i) the distance from the surface atthe point of contact to the center of curvature of the surface at thatpoint to (ii) the radial dimension of the roller, is not less than 3;

0. means for mounting the transfer roller so that it may move in adirection which is at least generally perpendicular to a tangent at thepoint of contact between the surface and the roller;

d. means for feeding a support sheet between the surface and the roller;and

e. means for biasing the roller against the support sheet with a minimalforce which is sufficient only to keep the roller in contact with thesupport sheet and the support sheet in contact with the surface duringmovement of the support sheet between the surface and the rollers duringoperation of the apparatus so that transfer can be effected solely by anelectrical bias applied to the transfer roller.

2. Apparatus as set forth in claim 1 wherein the ratio of (i) to (ii) isin the range 0f3 to 3.5.

1. Apparatus comprising: a. means defining a surface adapted to supporta developed electrostatic image, the surface being mounted for movementaround a closed path; b. a transfer roller assembly having a transferroller in contact with the surface, wherein the ratio of (i) thedistance from the surface at the point of contact to the center ofcurvature of the surface at that point to (ii) the radial dimension ofthe roller, is not less than 3; c. means for mounting the transferroller so that it may move in a direction which is at least generallyperpendicular to a tangent at the point of contact between the surfaceand the roller; d. means for feeding a support sheet between the surfaceand the roller; and e. means for biasing the roller against the supportsheet with a minimal force which is sufficient only to keep the rollerin contact with the support sheet and the support sheet in contact withthe surface during movement of the support sheet between the surface andthe rollers during operation of the apparatus so that transfer can beeffected solely by an electrical bias applied to the transfer roller. 2.Apparatus as set forth in claim 1 wherein the ratio of (i) to (ii) is inthe range of 3 to 3.5.